It is known that a laser device in which high power is essential generally uses a method of acquiring desired light intensity by focusing a plurality of laser beams onto a target simultaneously. Such a laser device divides or splits a laser beam emitted out of one master oscillator into a plurality of laser beams, amplifies them respectively, and focuses the amplified plurality of laser beams which are arranged as if they are bundled (referred to as the main output beam) onto a small target at a long distance in order to centralize the energy onto the target. Thus, a technology for focusing laser beams (this technology is called coherent coupling) to acquire an output beam equivalent to a single laser beam having a large diameter from a bundle of a plurality of laser beams has been studied.
In order to implement the coherent coupling, it is necessary to carry out control of the phases of the individual laser beams which construct the main output beam so that the equiphase wave surfaces of the electromagnetic waves of the plurality of laser beams can be assumed to be one wave surface. For this reason, a laser phase difference detecting device for detecting a relative phase difference among the phases of the individual laser beams is needed.
A wave front sensor as shown in FIG. 5 of patent reference 1 is an example of such a laser phase difference detecting device, and uses, as reference light, light into which it splits a part of a plurality of laser beams emitted out of a master oscillator which is a source for generating the plurality of laser beams using a beam splitter, and observes the intensity of interference between the reference light and the main output beam and detects the relative phase difference among the phases of the individual laser beams.
[Patent reference 1] JP,11-340555,A (see FIG. 5)
A problem with the prior art laser phase difference detecting device constructed as mentioned above is that because it needs to extract a part of the light emitted out of the master oscillator and to use it as the reference light, the device needs to make the reference light travel a long distance in a case in which the distance between a position where it generates the main output beam and the master oscillator is large, and therefore the device is enlarged and the manufacturing cost of the device increases.
Furthermore, when time fluctuations in the optical path length occur in the optical path in which the reference light is made to propagate due to atmospheric density change, vibrations, etc., the time fluctuations in the optical path length have an influence on the detection of the phase difference. Another problem is therefore that in this case, because the time fluctuation components in the length of the optical path of the reference light must also be taken into consideration and corrected in order to correct for the phase difference using the detected phase difference, the amount of calculations for the correction may increase and the system may not be stabilized. A further problem is that there is a necessity to additionally provide a means for detecting the time fluctuation components of the length of the optical path of the reference light to make a correction to the time fluctuation components, and therefore the manufacturing cost of the device increases.
A still further problem is that in a case of applying the prior art laser phase difference detecting device to coherent coupling used for generating a main output beam having a short coherence length of the order of about several 10 micrometers, like a super-short pulse laser beam, because the reference light and the main output beam interfere each other and an allowable range of an optical path length difference is small, it takes much time and effort to make an adjustment to the coherent coupling.
The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide a small-size, low-cost, and easy-to-use laser phase difference detecting device.
It is another object of the present invention to provide a laser phase control device using the laser phase difference detecting device.